driver list: use dynamic linking from automake build rules

driver list: use dynamic linking from automake build rules

Adjust the Makefile.am to dynamically link the "sigrok library" to the
application executable. Static linking fails because the linker script
provided start symbol for the drivers list is missing in the static .a
library.

One way of unbreaking the static build could be to provide a weak symbol
for the list start, though that would again be compiler specific. But it
would not unbreak the execution either, because the start of the driver
list section for static linking is still unknown. Which strenghens the
suspicion that the link time approach is less portable and reliable than
the constructors are.